Polysilicon Films Formed on Metal Sheets by Aluminium Induced Crystallization of Amorphous Silicon: Barrier Effect
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1153-A05-04
Polysilicon Films Formed on Metal Sheets by Aluminium Induced Crystallization of Amorphous Silicon: Barrier Effect P. Pathi, Ö. Tüzün and A. Slaoui Institut d'Électronique du Solide et des Systèmes, UMR 7163 CNRS-UdS, 23 rue du Loess, F-67037 Strasbourg Cedex 2, France
ABSTRACT Polycrystalline silicon (pc-Si) thin films have been synthesized by aluminium induced crystallization (AIC) of amorphous silicon (a-Si) at low temperatures (≤500°C) on flexible metallic substrates for the first time. Different diffusion barrier layers were used to prepare stress free pc-Si films as well as to evaluate the effective barrier against substrate impurity diffusion. The layers of aluminum (Al) and then amorphous silicon with the thickness of 0.27 µm and 0.37 µm were deposited on barrier coated metal sheets by means of an electron beam evaporation and PECVD, respectively. The bi-layers were annealed in a tube furnace at different temperatures (400-500°C) under nitrogen flow for different time periods (110hours). The degree of crystallinity of the as-grown layers was monitored by micro-Raman and reflectance spectroscopies. Structure, surface morphology and impurity analysis were carried out by X-ray diffraction, scanning electron microscopy (SEM) and EDAX, respectively. The X-ray diffraction measurements were used to determine the orientation of grains. The results show that the AIC films on metal sheets are polycrystalline and the grains oriented in (100) direction preferentially. However, the properties of AIC films are highly sensitive to the surface roughness. INTRODUCTION Among the alternative approaches for efficient thin film solar cells, polycrystalline silicon (pc-Si) thin films on foreign substrates (such as ceramic, graphite, glass) seem a very promising candidate due to their low cost and high efficiency potential [1]. Particularly, the use of flexible substrates paves a way for a new trend in the development and application of solar cells due to their unique advantages such as light weight and flexibility in different sorts like building integration and orienting on uneven surfaces. Cr free steel foils are more effective in terms of economic production of solar cell modules. In addition, it could be possible to exploit the inherent properties of metals like optical reflection and electrical conductivity in the solar cells made on metallic substrates. However, various key points need to be addressed when the metallic sheets are used as substrates, particularly with temperature processing. Some of the main issues are; diffusion of substrate elements like Fe, Ni, etc., which is detrimental for the life time of the charge carriers and hence the device efficiency, mechanical integrity of barrier layer with substrate as well as silicon films due to the mismatch of coefficient of thermal expansion and corrosion. The migration of impurities can be avoided by incorporating an intermediate layer called, “diffusion barrier”. The barrier performance against the migration of impurities depends on its physical characteri
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